Does Vallejo primer needs to be diluted with thinner? The reason I am asking it because I saw the AK-interactive tutorial video where the guy who pour the primer directly into the airbrush cup without adding thinner. But when I did the same thing my airbrush seems to clog badly....

What cone do you use? If you are using 0,35 or bigger there are no problems. I always use it out of the bottle

I mix it approximately 75% primer to 25% Vallejo Airbrush thinner and shoot just below 20psi.

My airbrush is an Iwata 0.3mm.

I usually add a small amount of thinner to my Vallejo primer to stop it drying too quickly, can't say any exact percentages though.

I concur with Kevin — give-or-take percentages.

I've learned to thin-down (primers included) with Vallejo Airbrush Thinner (71.161). As an added measure, I put in a drop or so of 'Liquitex Airbrush Medium.'

In addition to Vallejo airbrush thinner and Liquitex airbrush medium, you can also use Liquitex Flow-Aid and Liquitex Slo-Dri to keep things moving through your airbrush.

I know what's coming next. ... What's the difference between Liquitex Airbrush Medium, Liquitex Flow-Aid, and Liquitex Slo-Dri as mentioned above? I somehow settled on the first one but I've always been curious. Hope I'm not off-topic.